The present invention relates to lithium manganate which is a compound useful as an active material for positive electrode for lithium battery, a process for producing the same, a positive electrode for lithium battery using the same as an active material for positive electrode, and a lithium battery.
Lithium manganate is a compound represented by the formula LixMnyO4, and representative are spinel type LiMn2O4, Li4/3Mn5/3O4 and the like. For obtaining such lithium manganates, a process of firing a mixture of a manganese compound and a lithium compound at a temperature of about 800xc2x0 C. is employed.
Lithium manganate obtained by the above conventional process is apt to become a sintered body in which non-uniform sintering occurs between particles because a mixture of a manganese compound and a lithium compound is fired at about 800xc2x0 C. for the purpose of adjustment of valence of manganese or diminishment of by-products. Thus, there is a problem that size of particles cannot be controlled. Moreover, since a mixture of a manganese compound and a lithium compound is inferior in reactivity even when it is fired at high temperatures, uniform composition can hardly be obtained and there are many lattice defects. In order to avoid these problems, firing or mechanical grinding must be repeated many times.
Furthermore, lithium secondary batteries which use lithium manganate obtained by the above process as an active material for positive electrodes are not only low in initial charge and discharge capacity, but also show conspicuous reduction in capacity with repetition of charge and discharge. This is because crystals of lithium manganate are collapsed at the time of charging and discharging, which is considered to be caused by the presence of lattice defects and the low lithium ion conductivity.
For the solution of the above problems, there is proposed a process which comprises impregnating a porous manganese dioxide with lithium acetate, lithium nitrate or lithium hydroxide and obtaining a product of uniform composition at low temperatures (for example, xe2x80x9cElectro-chemistry (Denki Kagaku)xe2x80x9d, 63, 941 (1995)), but this process is still not sufficient.
The inventors have conducted intensive research in an attempt to obtain lithium manganate useful as active materials for positive electrodes of lithium batteries. As a result, it has been found that lithium secondary batteries in which is used an active material for positive electrodes which comprises lithium manganate having a cubic particle form and containing voids in the particles are high in initial charge and discharge capacity and excellent in cycle characteristics after repetition of charge and discharge. After additional investigations, the present invention has been accomplished. That is, the present invention relates to a lithium manganate which has a cubic particle form and provides an initial discharge capacity of at least 95 mAh/g when it is used as an active material for positive electrodes of lithium batteries. Furthermore, the present invention relates to a process for the advantageous production of the lithium manganate, and the first process is characterized by including a step of reacting a manganese compound with an alkali to obtain a manganese hydroxide, a step of oxidizing the hydroxide in an aqueous medium or a gaseous phase to obtain a manganese oxide, a step of reacting the manganese oxide with a lithium compound in an aqueous medium to obtain a lithium manganate precursor, and a step of firing the precursor with heating to obtain lithium manganate. The second process is characterized by including a step of reacting a manganese compound with an alkali to obtain a manganese hydroxide, a step of oxidizing the hydroxide in an aqueous medium or a gaseous phase to obtain a manganese oxide, a step of reacting the manganese oxide with an acid in an aqueous medium to substitute proton for a part of manganese to obtain a proton-substituted manganese oxide, a step of reacting the proton-substituted manganese oxide with a lithium compound in an aqueous medium to obtain a lithium manganate precursor, and a step of firing the precursor with heating to obtain lithium manganate. The present invention further relates to a positive electrode for lithium battery in which the above lithium manganate is used as an active material for the positive electrode and to a lithium battery using the positive electrode.